In Stevens U.S. Pat. Nos. 3,485,234 and 3,585,707, tubular products such as catheters are disclosed which comprise an extruded plastic coating having a tubular, braided wire sheath disposed tightly about a plastic coating in telescoping relation therewith. A second, extruded layer of plastic is then applied over the braided sheath.
Such a catheter, having a braided sheath, can increase both the longitudinal and the torsional stiffness of the catheter. Thus, at least a main portion of the catheter body may be longitudinally stiffened to facilitate the insertion of arteriovenous catheters, for example. An improved torsional stiffness permits torque to be better transmitted from the proximal end of the catheter to typically the distal tip. Such a characteristic is desirable to facilitate advancement of, for example, an intravascular catheter through a branching blood vessel system of a patient. As the catheter is advanced, the surgeon may rotate the proximal end, and, when a catheter has improved torsional stiffness, the distal end 20 more accurately follows the desired rotational movement imparted to the catheter by the surgeon.
The braided wire which is currently applied to catheters may increase or decrease both the longitudinal stiffness and the torsional stiffness in a manner which varies with the angle that the crossing strands exhibit to each other and to the axis of the catheter. However, the longitudinal stiffness increases when the strand angles to the catheter axis are reduced while the torsional stiffness increases with increasing angles of the strands to the catheter axis. Thus, one must sacrifice torsional stiffness in order to obtain good longitudinal stiffness while one must sacrifice longitudinal stiffness in order to get good torsional stiffness, depending upon the angles of the strands.
By this invention, an improved tubular sheath is provided to catheters, in which the sheath is made of helically disposed crossing strands in a manner analogous to the prior art, but with differences in the structure which permits increases in both the torsional and the longitudinal stiffness of the catheter in a desired but controllable manner. Thus, catheters of new and desired characteristics may be produced by this invention, which catheters were effectively unavailable in accordance with the techniques of the prior art.